1. Technical Field
The present invention relates to a transmission modulation apparatus that utilizes polar modulation and is useful in high efficiency linear transmission modulation system.
2. Description of the Related Art
Generally, in the conventional configuration of linear transmission modulation apparatus, there is a tradeoff relationship between efficiency and linearity. However, recently, various techniques are proposed that make possible both high efficiency and linearity in linear transmission modulation apparatus by employing polar modulation.
FIG.1 is a bock diagram showing the configuration of a conventional transmission modulation apparatus employing polar modulation. In this configuration, baseband amplitude modulation signal S1 is inputted in power source voltage controller 11, and its output and high frequency phase modulation signal S2 are combined in high frequency power amplifier 12 and give linear, high frequency transmission modulation signal S3 (output transmission signal).
Power source voltage controller 11 is often implemented using a switch mode power source having a class D amplifier in the output stage for maximum efficiency. Usually, a switch mode power source is implemented utilizing pulse width modulation, and the output of such power source is given in the form of a rectangular wave where the high/low (high level/low level) ratio represents a baseband amplitude modulation signal.
However, when amplitude modulation is performed utilizing pulse width modulation such as mentioned above, this creates intermodulation distortion in high frequency output transmission signals. A shown in FIG. 2, a solution to the above problem is to configure a power source voltage controller with delta amplifier 21, delta amplify baseband amplitude modulation signal S1 and supply the result to high frequency power amplifier 12, delta amplify the switch mode power source voltage, and improve the distortion of the high frequency transmission modulation signal by the negative feedback loop of the delta amplification. See, for example, Japanese Patent Publication No. HEI 10-256843 (Paragraphs [0019]-[0023], FIG. 3).
However, delta amplification does not process the DC (direct current) component, and so it is not possible to output fixed voltage (DC component) from the power source voltage controller. That is, the use of delta amplification makes it difficult to give fixed voltage as the power source voltage of high frequency power amplifier. Consequently, for example, if one tries to implement a transmission modulation apparatus that supports various modulation schemes, it is not possible to share a power source voltage controller if there are schemes whereby the amplitude modulation signal does not fluctuate (e.g., GSM scheme). In addition, if amplitude modulation needs to be performed in an early stage of the high frequency power amplifier, the high frequency power amplifier needs to switch from switching operation to linear operation. It is then difficult to give fixed voltage as the source for the high frequency power amplifier.
As shown in FIG.3, a solution to the above problem is to configure a power source voltage controller with delta sigma amplifier 31, delta sigma amplify baseband amplitude modulation signal S1 and supply the result to high frequency power amplifier 12, and make it possible to transmit the DC component. See, for example, Japanese Patent Publication No.2002-530917 (Paragraphs [0018]-[0024], FIG. 1).
However, the above-mentioned transmission modulation apparatus utilizing delta sigma amplification has the problem that noise concentrates in high frequency domain due to the noise shaping characteristics of delta sigma amplification and the problem of deterioration in spectral characteristics such as adjacent channel leakage power.